1. Field of the Invention
This invention relates to printing processes, more particularly to screening processes for images to be printed.
2. Background of the Invention
There are different printing technologies like electrophotography, dye sublimation, ink jet, offset, etc. Although dye sublimation technology can realize significant number of gray levels per pixel, other technologies realize fewer. For example, a spatial light modulator based electrophotographic printer can realize between 16-32 gray levels per pixel. In order to simulate the appearance of continuous tone (contone) on these printers it is necessary to use an approach called multi-level screening. Multi-level refers to multiple level of gray shades realized by these printers, but the numbers of levels is much less than 256, which is typically required to reproduce contone data.
As an example of multi-level printing, suppose an image is to be printed on a 600 dpi (dots per inch) printer that can only reproduce 16 levels (4 bits) of gray. The page containing the image can be logically divided into 4 pixels wide--4 pixel high blocks (cells) that completely tile the page. Each pixel in the cell can represent one of 16 shades, the cell itself can totally simulate 241[(16-1).times.4.times.4+1] gray shades. For example, filling a pixel with level 15 and another with level 10 would result in a simulated gray shade of 25. Several techniques can be used to decide the order in which pixels in a cell are filled.
Digital data compression is a technique that is used in systems (including printers) when there are constraints on available memory or in bandwidth. Typically, digital compression methods work on contone data. In printers, compression methods can be used to reduce the amount of frame buffer memory required when printing a page. The input to a printer is a page description language (like PostScript, PCL, etc.) which is converted by a processor called a Raster Image Processor (RIP) to a pixel map in a frame buffer.
When memory available is limited, the output bitmap is generated a band at a time and compressed. As an example, for a 81/2".times.11", 600.times.600 dpi printer with 4 bits per pixel and 4 color planes, the total memory is required is 64 Mbytes. However, if the RIP generates a 1/2" band at a time, compresses it by a factor of 8 and stores it, the effective memory required is 64/8+Memory for 1 band which equals 8+11.times.600.times.600.times.4/8, or 8.95 Mbytes (assuming that a band is in the 11" direction and 1/2" tall). This is significantly less than the memory required for a full frame buffer system.
Further, in full frame buffer systems compression may be required if the data link from memory to the marking engine is of limited bandwidth. However, no effective method of compressing multi-level data currently exists. Clearly, the need for one exists.